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The Case of the Missing Y Chromosome

Sometimes, a fertilized egg starts out as male (XY), but during the early cell division, one twin loses the Y chromosome. You’re left with one twin who is XY (male) and another who is X0—which is a condition called Turner syndrome, where the person develops as female. Wait, come back—I promise this isn’t a weird medical lecture. It’s actually a real thing that’s happened, like, a handful of times in recorded medical history.

That twin is genetically different from their sibling, so they’re not “pure” identicals anymore, but they started from the same egg. So you get a boy and a girl who look like identical twins, but with a chromosomal hiccup.

Can Identical Twins Be Male and Female? – About Twins

The Swyer Syndrome Twist

Then there’s an even rarer case: both twins are XY (genetically male), but one twin has a mutation that stops their body from responding to male hormones. Their body develops as female, even though their chromosomes say “male.” This is called Swyer syndrome.

So you could have a brother and a sister who are genetically identical males, yet one presents as a woman. Mind. Blown. It’s the ultimate biological irony. You two share the same DNA, but one of you gets a beard and the other gets a ponytail.